Many people, including me, thought the ubiquitous Hard Disk Drive (HDD) was destined for the tech scrap heap. Pundits say Solid State Drives (SSDs) perform better and are faster. Moreover, if prices continue to drop, the issue of cost disappears. Some experts urge caution.

Paula Long, CEO and cofounder of DataGravity, has been warning us about jumping ship too early for several years. I first learned of her appeal for caution back in 2013. "If you need to go zero to 60 in 2 seconds, flash is probably a great way to get there," Long explained in this Gigaom post by Derrick Harris. "If you don't need to go in 2 seconds, it's probably an expensive solution for what you need. Storage has never been one-size-fits-all, and it never will be."

A lot can change in two years

DataGravity CEO Paula Long

Image courtesy of DataGravity

More than a few tech-media outlets are now predicting SSDs will surpass HDDs in capacity and achieve parity in pricing by the end of 2016. Jim O'Reilly at InformationWeek's NetworkComputing wrote, "If you were told that a BMW and a golf-cart were the same price, which would you buy? That's going to be the dilemma facing buyers sometime in 2016, with SSD and HDD. I think I know your answer!"

I was curious to learn what Long thought about recent predictions from the tech media. Long began our conversation by saying the storage-technology landscape is not as predictable as most perceive. "It's not either/or — most modern storage arrays will have some SSDs," she said. "Doing so affords easy implementation and improved performance for applications requiring high-access rates."

Regarding access rates

Data access rates are measured using Input/Output Operations Per Second (IOPS). "SSDs almost always outperform NL SAS HDDs," said Long. However, she contends few environments need the kind of performance an all-SSD array provides. For example, if a typical environment is driving 30,000 IOPS, being able to do the same operations 10 times faster using an all-SSD array may be interesting, but not a requirement.

(Long explained there are three types of enterprise-class HDDs: NL SAS 7200 RPM, NL SAS 10K RPM, and NL SAS 15K RPM. The important thing to understand is that, the faster the rotation speed, the more reads/writes that can be done per second per drive.)

"There's also the argument that using SSDs reduces response time to applications," said Long. "This can be true, but well-built hybrid arrays (both HDDs and SSDs) can come close and at a lower cost."

What about cost?

Storage-device costs are measured in dollars/GB or dollars/TB. "HDDs are significantly less expensive than SSDs in terms of dollar/physical capacity available," said Long. "As long as the price of one media is lower than the other, it will remain an option."

Long also advised caution when vendors say SSDs are price competitive with HDDs, mentioning, "What they're saying is, if you have the perfect deduplication ratio and the perfect app, it gets close."

What does deduplication have to do with it?

Some applications in storage have a lot of duplicate data — virtual machines, for example. Multiple virtual machines running on the same server could mean close to 75% of the storage footprint is the same (i.e., duplicate data). "Modern storage arrays can save space by using a technique called deduplication where the applications share common data," explained Long. "This results in needing less physical space to store the same information."

Deduplication can be done whether using SSDs or HDDs. "However, the comparisons for cost have a 5-10x deduplication ratio for SSD and none for HDD," said Long. Another example would be applications that have little-duplicated data. SSDs are unable to compete with HDDs cost per GB or TB without deduplication.

Long reiterated, "When you are comparing costs, make sure vendors are comparing 'apples to apples.'"

Others agree with Long

Lucas Mearian of ComputerWorld agrees with Long in his June 23, 2015 post: The rise of SSDs over hard drives, debunked. "In spite of a recent report to the contrary, solid-state drives (SSDs) will not surpass hard disk drives (HDDs) in either price or capacity anytime soon, according to industry analysts," wrote Mearian.

Mearian quoted John Monroe, vice president of research for data center systems at Gartner, "There's a stupefying quantity of hogwash out there. HDD makers are forecasting a 20TB HDD...in 2020; my guess is the [manufacturer] cost would be around $175 per drive. In 2020, let's assume a cost of $0.11/GB for a 25TB SSD, that would be $2,750 manufacturer cost per drive."

Long reminded me, "As long as the price of one media is lower than the other, it will remain an option."